1. Field of the Invention
The present invention is used for image forming apparatuses such as copying machines and printers using the electrophotographic method and the electrostatic recording method, and relates to a developing apparatus for developing an electrostatic image on an image bearing body, a developer bearing body used for this developing apparatus and a production method for the developer bearing body.
2. Related Background Art
Conventionally, in an image forming apparatus of, for example, the electrophotographic type, an electrostatic latent image has been formed on an image bearing body made of electrophotographic photoreceptor, and the latent image has been developed by a developer unit. The developer unit has a development sleeve as a developer bearing body for bearing developer to convey.
The surface of this development sleeve is unevenly roughened to promote the conveyance of developer, and there are known knurled grooves in a development sleeve mainly for two-component development as shown in, for example, Japanese Patent Application Laid-Open No. 54-79043 formerly, and blasting treatment in a development sleeve mainly for one-component development as shown in Japanese Patent Application Laid-Open No. 55-26526.
In the case of a development sleeve subjected to blasting treatment, since asperities on the surface are prone to be worn and reduced by long-term use, for example, high-hardness material such as SUS (Vickers hardness Hv.congruent.180) is most frequently used for the material for the development sleeve in order to prevent the wear, and formerly the Alundum blasting method has been used in which alumina particles are used as blasting abrasive grains (Japanese Patent Application Laid-Open No. 57-66455).
As shown in Japanese Patent Application Laid-Open Nos. 57-116372, 58-11974, 1-131586 and the like, however, a sharp, uneven roughened surface is formed on the surface of a development sleeve made of SUS in the blasting treatment using Alundum. FIG. 14 is a schematic view showing a roughness profile curve for the development sleeve surface subjected to the Alundum blasting treatment. During long-term use, it is known that specially-fine toner and the like will be filled in sharp concave portions on this surface (hereinafter, the state in which this toner and the like are filled in will be referred to as "sleeve contamination"), and charging of toner in the portions will be hindered to cause a defective image.
Thus, a method of performing blasting treatment using spherical particles such as, for example, glass beads is considered. FIG. 15 is a schematic view showing a similar roughness profile curve using the glass beads blasting treatment. As shown in FIG. 15, the roughened surface having a smooth cross-sectional shape on the surface of the development sleeve of SUS can be obtained according to the glass beads blasting treatment, and the sleeve contamination can be reduced.
On the other hand, as the material for the development sleeve, aluminum is popularly being used. This is because if aluminum is used, the cost of the sleeve could be reduced although SUS is expensive, and if an a-Si drum (amorphous silicon drum) is used as a photosensitive drum, the aluminum sleeve will be indispensable for the following reason.
When the a-Si drum is used as a photosensitive drum at high humidities, an electric discharge product (such as NOx) adhering to the surface of the photosensitive drum takes up moisture so that surface charge on the photosensitive drum which forms an electrostatic latent image after charging and exposure escapes in the vicinity through the electric discharge product to disturb the latent image, resulting in a turbulent image. In order to prevent this turbulent image, there is a method in which the surface is made easier to be shaved like an OPC drum and the surface layer including NOx is shaved. This method is effective for the flow of the image, but the life of the a-Si drum will be naturally shortened. Thus, a surface-like heating element or the like is placed into the photosensitive drum, and it is heated while the image forming apparatus is in a standby state, to prevent the electric discharge product from taking up moisture. However, the heat at the photosensitive drum is transmitted to the development sleeve which is opposed thereto. If it is made of SUS having low thermal conductivity, the development sleeve will be considerably thermally deformed. When it is the first copy after the standby and for example, a halftone image which ought to have uniform density is copied, a defective image occurs as sleeve pitch-shaped unevenness in the density. In contrast, the development sleeve made of aluminum is hardly thermally deformed, and such unevenness in the density as the deformation is made conspicuous hardly appears. Therefore, it is indispensable to combine the aluminum sleeve with the a-Si drum (with a built-in heater).
Since, however, the aluminum sleeve has as low hardness as Hv.congruent.100, the asperities on the surface provided by the blasting treatment will be easily worn by use and reduced soon.
For this reason, as shown in Japanese Patent Application Laid-Open No. 1-276174, there is a carbon-coated development sleeve having high-hardness resin coated on the surface of the aluminum sleeve. As high-hardness resin, for example, phenolic resin is coated on the surface of the aluminum sleeve, and graphite is dispersed on the phenolic resin in advance to thereby obtain the conductivity required as the development sleeve.
In the carbon-coated sleeve, the phenolic resin is coated with a thickness of about 10 to 20 .mu.m by dipping or spraying, and therefore, the resin surface basically takes over the uneven shape of the aluminum surface as the substrate. The minute surface property, however, looks as if graphite particles 102 are imbedded in the phenolic resin 100 as shown in FIG. 16, and the roughness cross-sectional shape is comparatively close to the surface state subjected to the Alundum blasting treatment shown in FIG. 14, having a surface on which sharp asperities are present. Toner is imbedded in these sharp concave portions to easily cause the sleeve contamination.
This carbon-coated sleeve has conventionally been used for a developer unit for laser beam printers (LBP) for negatively chargeable OPC, digital copying machines and the like. In the case of LBP, long-term use is not assumed because the development sleeve is also included in a cartridge as consumables. The development is of the reversal development system using negative toner. The resin used as negative toner such as, for example, styrene acryl and polyester is basically strongly negatively chargeable. The negative toner is highly electrified, and the toner can have a sufficient amount of charge even if the sleeve contamination occurs, and therefore, there were many cases where almost no problem is presented. Also, since the carbon-coated sleeve is also shaved little by little, it may be considered that the contaminant also might have been shaved together. For the reason, however, the carbon-coated sleeve was inferior to SUS in respect of life although it has high hardness.
In recent years, however, there has been the tendency to further reduce the toner particle diameter in order to improve the image quality, and it could be understood that the sleeve contamination is prone to occur more than before.
With reference to FIG. 17, this will be described. FIG. 17 is a view obtained by enlarging the asperities in the roughness profile curve of FIG. 15. FIG. 15 is, as described above, the roughness profile curve obtained when the surface of the development sleeve made of SUS is subjected to the blasting treatment using spherical particles of glass beads. In FIG. 17, in the case of large-diameter toner, it does not enter cracks in large asperities in the roughness profile curve, i.e., small concave portions such as, for example, concave portions a, b and c, but if the toner is turned into smaller-diameter, it is considered that toner which enters those small concave portions a, b and c, and the like, will be increased to thereby cause the sleeve contamination.
In small-diameter toner having particle size distribution for an average particle diameter of, for example, 7 .mu.m, there is contained 15 to 20% of smaller toner having particle diameter of 4 .mu.m or less, and these toner enter small concave portions a, b, c and the like. Of course, if fine powder in the toner is removed, it will be possible to reduce smaller toner, but smaller toner cannot be reduced to 0% in the manufacturing cost of the toner.
Also, even if the particle diameter of the toner is not reduced as described above, if toner having low electrification property (particularly, positive toner) is used, slight sleeve contamination easily causes inhibited electrification of toner, resulting in a problem of low density.
Under such circumstances, it becomes necessary to take a countermeasure against the sleeve contamination in order to extend the life of the developer unit.